Field of the Invention
The field of this invention relates to dispersions of particles in a liquid medium. More particularly, the field of this invention relates to dispersions of natural or synthetic graphite dispersions-in-oil which are suitable as a constituent of a lubricating composition.
U.S. Pat. No. 2,176,879 (1939) of F. E. Bartell discloses a composition comprising a dispersion of colloidal or finely divided graphite in a non-flocculated form in an organic liquid. A strongly absorbed stabilizing agent is disclosed to prevent flocculation.
U.S. Pat. No. 4,203,728 (1980) of Richard B. Norton discloses a method for preparing a slurry comprising oil and coal. Suspension stability of the coal particles in the oil is achieved by surface oxidation of the coal particle prior to preparation of the slurry. Surface oxidation of the coal could be achieved by techniques disclosed in Supplemental Volume of the text, "Chemistry of Coal Utilization" edited by H. H. Lowry, John Wiley and Son, N.Y. (1963). One of the techniques involved a fluidized bed of air and coal. Another, involved contacting coal particles with aqueous solutions of alkaline permanganate, hydrogen peroxide, ozone solutions and the like. Air oxidation in the range of 120.degree.-350.degree. C. was disclosed as an effective way to oxidize the surface of such coal particles.
Efforts to prepare stable graphite-in-oil dispersions by fluidized bed oxidation in air were not successful. By stable is meant, that the particles do not flocculate and/or settle out upon standing. To facilitate the process of settling, we used an International Model BE Centrifuge. A 100 ml. sample of the graphite dispersion was placed in the centrifuge and centrifugated at 900 RPM for 18 hours. A 5 ml. sample was syphoned out and its graphite content determined. From this, the percent of graphite retained was calculated and used to compare the stability of different dispersions. Stability as determined by this method is essential to the provision of an economical yield of stable graphite dispersions when the product of the wet grinding step is subjected to centrifugation or settling to remove undesirably large (e.g., greater than 6 microns) particles. It is also necessary to ensure, for example, that the lubricating properties of the graphite containing lubricating oil is not lost due to separation of graphite from the lubricating oil during long periods of storage, and that no graphite particles or their flocs will plug up pores of oil filters and lubrication passages.
The requirements for a graphite-in-oil dispersion for use in lubrication are more severe than the requirements for a coal-oil dispersion whose main purpose is transport through a pipeline. Since movement in a pipe helps maintain the coal suspended, the requirements for a coal slurry are likely to be different from those of a graphite in-oil-dispersion.
Accordingly, a method to prepare a stable graphite in-oil-dispersion which is an improvement over the methods previously known is needed.